1. Field of the Invention
The present invention relates to a spark plug for use in igniting an internal combustion engine, and more particularly to a spark plug in which a packing is interposed between an insulator and a metal shell.
2. Description of the Related Art
Generally, in a spark plug for use in igniting an internal combustion engine such as an automobile engine, a cylindrical insulator is inserted and held in a cylindrical metal shell. A center electrode for forming a spark discharge gap in opposition to a ground electrode is welded to a leading end side of the metal shell, as well as a terminal electrode for applying a high voltage across the center and ground electrodes, are inserted in an axial hole formed in the insulator. The spark plug is mounted in an internal combustion engine such that a leading end (spark discharge gap) of the spark plug faces the interior of the combustion chamber.
The aforementioned insulator is inserted from a rear end side of the metal shell toward a leading end side thereof. The insulator is fixedly crimped between a rear end of the metal shell and a stepped portion formed on an inner peripheral portion of the metal shell such that a shoulder portion formed on an outer peripheral portion of the insulator is retained by the stepped portion. An annular plate packing is interposed between the stepped portion of the metal shell and the shoulder portion of the insulator so as to maintain airtightness therebetween (e.g., refer to JP-A-2005-190762).
A metallic material whose hardness is lower than that of the metal shell is generally used as the material of the plate packing. When the metal shell is crimped as described above, the plate packing undergoes crush deformation and assumes a state in which it is in close contact with both the metal shell and the insulator. For example, in a case where the hardness of the metal shell is 200 Hv to 300 Hv, a plate packing whose hardness is 180 Hv or thereabouts is used. As a result, the gap between the metal shell and the insulator is set in a closed state, thereby ensuring airtightness of the combustion chamber.
In recent years, in conjunction with trends toward higher power and fuel savings in internal combustion engines, miniaturization and development of smaller diameter spark plugs is underway. In the case where a small-diameter spark plug is fabricated, the wall thickness of the metal shell also becomes thin, so that if the crimping load is large, the strength of the metal shell declines. Hence, there is a possibility that the stepped portion formed on the inner peripheral portion of the metal shell and oriented toward the rear end becomes excessively deformed, to thereby impart a large eccentricity. On the other hand, if the load is made small to prevent the occurrence of this problem, it frequently becomes difficult to ensure airtightness. For this reason, a plate packing having a relatively low hardness is used so that the plate packing is deformed even by a small crimping load, to thereby allow the plate packing to be brought into close contact with the metal shell and the insulator.